1. Field of the Invention
The present invention relates to polyesters useful for facilitating the separation of blood serum or plasma from the cellular portion of blood.
2. Description of the Related Art
The compositions of the present invention are conveniently formulated into a partitioning composition for use in a blood collection vessel in which the blood sample is subjected to centrifugation until the cellular portion and serum or plasma are completely separated.
Note that while blood is the most usual candidate for physiological separation, conceivably urine, milk, sputum, stool solutions, meconium, pus and the like could all be subject to physiological separation and assay for therapeutic agents and the subsequent discussion, while focusing on blood for clarity, is not meant to be limited to blood.
The physical and chemical properties of the partitioning composition are such that a continuous, integral seal is provided between the separated blood phases, thereby maintaining separation of the phases after centrifugation and simplifying removal of the serum or plasma from the blood collection vessel. The high volume testing of blood components in hospitals and clinics has led to the development of various devices to simplify the collection of blood samples and preparation of the samples for analysis. Typically, whole blood is collected in an evacuated, elongated glass tube that is permanently closed at one end and sealed at the other end by a rubber stopper having a diaphragm which is penetrated by the double-tipped cannula used to draw the patient's blood. After the desired quantity of blood is collected, the collection vessel is subjected to centrifugation to yield two distinct phases comprising the cellular portion of the blood (heavy phase) and the blood serum or plasma (light phase). The light phase is typically removed from the collection vessel, e.g., via pipette or decantation, for testing. It has been proposed heretofore to provide manufactured, seal-forming members, e.g., resilient pistons, spools, discs and the like, in blood collection vessels to serve as mechanical barriers between the two separated phases. Because of the high cost of manufacturing such devices to the close tolerances required to provide a functional seal, they have been supplanted by fluid sealant compositions. Fluid sealant compositions are formulated to have a specific gravity intermediate that of the two blood phases sought to be separated, so as to provide a partition at the interface between the cellular and serum phases. Such compositions typically include a polymer base material, one or more additives for adjusting the specific gravity and viscosity of the resultant composition, and optionally, a network former. Representative fluid sealant compositions developed in the past include: styrene beads coated with an anti-coagulant; silicone fluid having silica dispersed therein; a homogeneous, hydrophobic polyester including a suitable filler, e.g., silica; a liquid alpha-olefin-dialkylmaleate, together with an aliphatic amine derivative of smectite clay or powdered silica; the reaction product of a silicone fluid with a silica filler and a network former; and a mixture of compatible viscous liquids, e.g., epoxidized vegetable oil and chlorinated polybutene, and a thixotropy-imparting agent, e.g., powdered silica, and liquid polyesters, a thixotropic gel comprising a dual resin component including poly-alpha-pinene of lower density combined with chlorinated octadecene of higher density, said gel further comprising a radiation stabilizer, a network stabilizer, a thixotropic agent and a pigment, and a gelatinous material admixed with fine resin particles having an average particle size of 0.01 to 2 microns and having an internal crosslinking density of 0.1 to 3 mmol/g.
Ideally, a commercially useful blood partitioning composition should maintain uniform physical and chemical properties for extended time periods prior to use, as well as during transportation and processing of blood samples, readily form a stable partition under normal centrifugation conditions and be relatively inert or unreactive toward the substance(s) in the blood whose presence or concentration is to be determined.
Inertness to substances sought to be determined is a particular concern when blood collection vessels are used for therapeutic drug monitoring (TDM), which is assuming an increasingly important role in drug treatment strategies. TDM enables the administration of drugs in the appropriate therapeutic ranges, established through the accumulated experience of clinicians, and consequently reduces the number of patients receiving dosage levels that are either below detection limits or toxic. Administration of drugs under TDM allows one to take into account such factors as drug tolerance developed with passage of time, presence of multiple physical disorders and synergistic or antagonistic interactions with other therapeutic agents. Among the drugs recommended for administration under TDM are those having dangerous toxicity with poorly defined clinical endpoint, steep dose-response curve, narrow therapeutic range, considerable inter-individual pharmacokinetic variability or non-linear pharmacokinetics, as well as those used in long term therapy or in the treatment of life-threatening diseases. By way of example, the evaluation of blood levels of a number of tricyclic antidepressant compounds, such as imipramine or desipramine, in relation to an empirically established therapeutic range is reported to be particularly useful in the treatment of seemingly drug-refractive depression. TDM is likewise used to monitor the dosage of anticonvulsant drugs, such as phenytoin and phenobarbital which are administered in the treatment of epilepsy, anti-tumor drugs, such as methotrexate, and other more commonly prescribed drugs, including, but not limited to digoxin, lidocaine, pentobarbital and theophylline.
Reports of studies on the effect of blood partitioning compositions on drug concentrations in serum and plasma indicate that care must be taken in the selection of polymeric materials which come into contact with the blood samples obtained for drug assay. See, for example, P. Orsulak et al., Therapeutic Drug Monitoring, 6:444-48 (1984) and Y. Bergquist et al. Clin. Chem., 30:465-66 (1984). The results of these studies show that the blood partitioning compositions provided in blood collection vessels may account for reduced serum or plasma values, as a result of drug absorption by one or more components of the composition. The reported decreases in measured drug concentrations appears to be time dependent. One report concludes that the observed decreases in drug concentrations may effectively be reduced by minimizing the interval between collection and processing. Another report recommends that blood samples be transported to the laboratory as soon as possible, with processing occurring within 4 hours. A commercially useful blood collection vessel, however, must produce accurate test results, taking into account routine clinical practices in large institutions, where collection, transportation and processing of blood samples may realistically take anywhere from about 1-72 hours.
Conventional polyester fluids are inadequate penetration barriers and therapeutic agents will diffuse into them and be partially absorbed with time, which interferes with quantitative assay for their presence. Attempts to solve this problem have centered around techniques for making the polyester itself more hydrophobic. Most therapeutic agents have high solubility parameters and associated high hydrophilicity (associated with high polarity functional groups like amines) because they must be soluble in aqueous liquids like blood, and water has a high solubility parameter. So, the direction has been to less hydrophilic, more hydrophobic, polymers to avoid the possibility of the therapeutic agents partially dissolving in a medium solubility parameter, medium polarity, polyester, and thus be more fully available in the serum phase for analysis. Imparting hydrophobic character to the polyester has been done via two main techniques. Firstly, a random polymer has been made of a diol and large quantities of a dicarboxylic acid with pendent, long (C.sub.9 to C.sub.13) olefin groups. Secondly, a random polymer has been made of a diol and large quantities of a dicarboxylic with a long olefin along its backbone, such as a C.sub.36 dimerized fatty acid. Such polyester compositions have proved useful as functional blood partitioning compositions having reduced affinity for therapeutic agents present in blood such as phenobarbital and imipramine. See, for example, W. L. O'Brien, U.S. Pat. No. 5,124,434, the entire disclosure of which is incorporated by reference in the present specification, as if set forth herein in full.
However many of these polyesters are highly viscous and difficult to transfer to the sample collection vials. It is therefore desirable to have a material that will facilitate physiological separations but does not have the difficulties associated with transferring highly viscous liquids.